Nitrogen oxide (NO.sub.x) emission control has been the focus of recent legislative initiatives in the United States, Canada, Europe and the Pacific Rim region. In addition to being a primary precursor of acid rain, NO.sub.x plays a minor role in ozone pollution. One-third of all man-made NO.sub.x emissions in the U.S. in 1985 were from automobiles, according to the EPA Publication, "Anthropogenic Emission Data for the 1985 NAPAP Inventory" (EPA600/788022). In the early 1980's the automotive industry started adding rhodium to their exhaust catalysts to aid in the reduction of NO.sub.x emissions. Engines were controlled to operate at the stoichiometrically balanced air-to-fuel ratio, A/F=14.4-14.7, because at this ratio three-way (Pt--Rh) catalysts simultaneously promote the conversion of the three primary exhaust pollutants: CO, hydrocarbons, and NO.sub.x. Recently, automobile manufacturers have developed lean-burn spark-ignition engines which operate at A/F/ratios of 18-23 because they are more fuel-efficient, have longer life, and produce fewer pollutants than stoichiometric burn engines. The exhaust from lean-burn engines is oxygen-rich and contains lower levels of hydrocarbons and carbon monoxide. The implementation of lean-burn engines is limited by the inability of the three-way catalyst to remove NO.sub.x from the oxygen-rich exhaust. Furthermore, legislation is being proposed to limit NO.sub.x emissions from diesel engines (e.g. automotive, construction, farm and marine) which are also lean-burn engines.
The largest single source of NO.sub.x emissions, approximately 50% of the total, is burners and stationary engines in utility and industrial complexes, according to the EPA. Legislative mandates have lowered the permitted levels for these NO.sub.x emissions to a point where Selective Catalytic Reduction (SCR) units are increasingly being required to meet NO.sub.x emission specifications. The standard catalysts for SCR applications are based on vanadium with a titanium oxide matrix, supported on various types of monoliths. There is growing concern in the petroleum refining community about the long term viability of this type of catalyst due to its restricted window of temperature operability and disposal problems due to the metals.
In response to these challenges, a large amount of research has been carried out, inside academia and industry, on alternate catalyst systems for SCR and oxygen-rich engine exhaust (lean burn gasoline and diesel) applications. A large portion of this work has focused on the potential of zeolites.
The so-called "stable" nitrogen oxides have in common the somewhat peculiar property that although they are thermodynamically unstable with respect to decomposition into elemental oxygen and nitrogen, no simple, economical method has been described for inducing this decomposition. It has been discovered, however, that the addition of a reductant such as ammonia or an ammonia precursor to the exhaust gas, under appropriate reaction conditions, converts NO.sub.x to elemental nitrogen and steam and denitrifies the exhaust gas.
The process of contacting an industrial flue gas with a catalyst in the presence of ammonia at a temperature in the range of about 200.degree.-600.degree. C. to denitrify the flue gas has come to be known as the process for Selective Catalytic Reduction (SCR) of NO.sub.x. In order to avoid confusion, any reference made herein to "Selective Catalytic Reduction," or to "SCR," is intended to refer to a process in which a mixture of NO.sub.x and NH.sub.3 are induced to react catalytically at elevated temperatures. The term "denitrify" as used herein, means to reduce the amount of one or more noxious nitrogen compounds (such as NO, NO.sub.x and HCN) contained in a waste gas, preferably by conversion to nitrogen gas. SCR is more particularly described in U.S. Pat. No. 5,589,147.
U.S. Pat. No. 5,589,147, herein incorporated by reference, discloses a process for Selective Catalytic Reduction of NO.sub.x in exhaust gas. NO.sub.x is reduced over a catalyst composition comprising a molecular sieve that has been treated with a metal in a in a way effective to maximize metal dispersion. The catalyst of this invention typically comprises a silica, titania, or zirconia binder.
U.S. Pat. No. 5,552,129 discloses an SCR process which employs a catalyst composition comprising a metal and an in-situ crystallized zeolite such as ZSM-5.
U.S. Pat. No. 5,254,322 is concerned with a method for reduction of nitrogen oxides contained in a gaseous stream such as lean burning internal combustion engine exhaust. The method employs a hydrothermally stable catalyst comprising transition metal-containing ZSM-5 which is prepared by in-situ crystallization of a preformed aggregate.
U.S. Pat. No. 5,254,322 discloses that nitrogen oxides contained in an exhaust gas from an internal combustion engine operating under lean burn conditions can be reduced by contacting the exhaust gas at a temperature of at least 300.degree. C. with a hydrothermally stable catalyst comprising a transition metal and a zeolite having the structure of ZSM-5 which is prepared by in-situ crystallization of an aggregate comprising ZSM-5 seeds, silica, and a crystalline silicate. The exhaust gas has a molar ratio of hydrocarbons to nitrogen oxides of at least the stoichiometric ratio, and the reduction of NO.sub.x is substantially effected by hydrocarbon reductant. It has been found that the catalysts employed in this invention are more heat and/or steam stable in this particular use than corresponding zeolite-containing catalysts prepared by other methods which enhance thermal and/or hydrothermal stability.